The invention relates generally to a tool for forming bores in relatively hard materials, and in particular to a rotary drill bit for use in oil, gas, and mining exploration that can maintain low impact and cool operating conditions which facilitates the drilling of harder and more abrasive formations.
Oil and gas field drilling and mining in general employ drills bits having hard and durable cutting contact surfaces. One such cutting and wear insert material used is polycrystalline diamond compact (PDC). PDC is an extremely hard and wear resistant material.
PDC cutters are known to have the lowest rates of wear when used in cooler operating conditions. Wear rates are low when the operational temperatures are maintained below about 700 degrees Celsius. At about 700 degrees Celsius, thermal damage to the diamond layer begins, resulting in loss of wear resistance. Above the critical temperature, the rate of wear of the cutter can be as much as fifty times greater than its rate at cooler conditions.
In conventional PDC drag bits, the velocity of a cutter on a drill bit, when measured relative to the material being cut, depends on its distance from the center of rotation of the drill bit. The further away the cutter is from the axis of rotation of the bit, the greater the velocity of the cutter. Thus, increasing the diameter of a drill bit results in greater velocity for the outside cutters. With greater velocity, there is greater friction, and thus greater heat generated during drilling. At some point, the heat will be sufficient to cause wear rates to accelerate, thus reducing the life of the outside cutters. Heating is particularly a concern for PDC cutters, as PDC tends to break down at elevated temperatures, resulting in a loss of wear resistance and increased breakage due to impact.
Furthermore, when more force is applied, more heat is generated. As harder rock requires more force for cutter penetration, wear rates will naturally be higher in such formations. The critical point at which the wear rate begins to accelerate is a function of rock hardness and bit rotational speed. In softer rocks, accelerated wear rates do not occur until higher rotational speeds are used. Whereas in harder rocks, acceleration of the wear rate occurs at much lower rotational speeds.
Another cause of wear to PDC cutters is breakage from impact.When drilling with conventional PDC drag bits, weight and torque are applied to a drill string. PDC cutters are driven into the formation by applied weight. Torque rotates the bit, dragging its PDC cutters through the formation. Dragging generates chips that are removed, thereby forming the hole. This drilling action causes a reverse, corresponding torque to the drill string. Because of the length of the drill string, the torque tends to wind it like a torsion spring. When conditions are not stable, this tends not to be a problem. But should the bit release from the formation, the drill string will unwind and rotate backward. The resulting load on the drill bit, when it hits against the formation can cause impact damage to the cutters. Furthermore, under typical drilling conditions, a drill string is rotated at 90 to 150 rpm. At these higher speeds, drill strings can tend to vibrate, sometimes severely. Vibration can damage a drill bit, including the cutters, as well as the drill pipe, MWD equipment, and other components in the drilling system.
Contributing to impact loads on PDC cutters is a phenomenon known as xe2x80x9cbit whirl.xe2x80x9d This complex motion of the drill bit is thought to be the result of a combination of causes, such as lateral forces from drill string vibration, heterogeneous rock formations, bit design, and other factors in combination with a radial cutting ability of PDC bits. When a drill bit begins to whirl, PDC cutters on the bit are subjected to large impact loads as the bit bounces against the rock. The cutters can lose large chips of PDC from impact rather than from gradual abrasion of the cutter, which thereby shortens bit life.
PDC cutters thus maintain the longest useful life when used under low impact and cool operating conditions, and in these conditions, they are able to cut extremely hard and abrasive materials with long life. Thus, the usefulness of such drill bits in hard formations tends to limited to low rotational speeds, and thus relatively slow rates of penetration in typical oil well drilling.
The invention is directed generally to an improved drilling tool and method for drilling. The invention, as defined by the appended claims, has various aspects and advantages that are described below with reference to an example of a drilling tool that embodies the invention.
This exemplary drilling tool includes several features that singularly and collectively can be used to reduce the adverse thermal and/or impact effects on cutters, extending the life of the cutters without affecting drilling performance, and thus also better enabling PDC cutters to be used in hard rock formations and other situations in which they typically have not been used due to such effects. Briefly, this exemplary drilling tool reduces the surface speed of outer cutters, thus reducing thermal stress on the cutters without reducing drilling effectiveness. Furthermore, the drillstring may be rotated at lower rotational speeds, producing less reactive torque, and keeping the drilling tool on a straighter path, thereby tending to reduce vibration, torque on the drill string and xe2x80x9cwhirlxe2x80x9d. To reduce complexity and improve reliability, the exemplary drilling tool utilizes abrasion-resistant bearing and gear surfaces, capable of carrying relatively large loads, thus avoiding the need for sealed bearings and gears and permitting use of drilling fluid for cooling and lubrication.